1. Technical Field
This application relates to computer storage devices, and more particularly to the field of transferring data between storage devices.
2. Description of Related Art
Host processor systems may store and retrieve data using a storage device containing a plurality of host interface units (host adapters), disk drives, and disk interface units (disk adapters). Such storage devices are provided, for example, by EMC Corporation of Hopkinton, Mass. and disclosed in U.S. Pat. No. 5,206,939 to Yanai et al., U.S. Pat. No. 5,778,394 to Galtzur et al., U.S. Pat. No. 5,845,147 to Vishlitzky et al., and U.S. Pat. No. 5,857,208 to Ofek. The host systems access the storage device through a plurality of channels provided therewith. Host systems provide data and access control information through the channels to the storage device and the storage device provides data to the host systems also through the channels. The host systems do not address the disk drives of the storage device directly, but rather, access what appears to the host systems as a plurality of logical disk units. The logical disk units may or may not correspond to the actual disk drives. Allowing multiple host systems to access the single storage device unit allows the host systems to share data stored therein.
In some cases, it may be desirable to perform a copy process by first specifying (and identifying) a set of extents on a source volume that are to be copied. Subsequently, the copy process is initiated with one or more targets. There is a risk that the copy command will copy the wrong data if the data is modified between the time the set of extents is specified and the time that the copy operation is initiated. Although it may be possible to provide write protection/indirect sessions for each of the sets of extents, such a mechanism may become impractical as the number of sets increases.
Accordingly, it is desirable to provide an efficient mechanism to determine if data has been modified between the time one or more sets of extents are specified and the time a copy operation is initiated.